You only have an Order ID number when your order goes through. In the case it doesn’t, just choose something else from the drop downs that doesn’t ask for an Order ID, like report a bug or general inquiries or something. You can tell them the username of the seller and send a link to the gig you were trying to buy instead of the Order ID then in the text field.
Usually, payment goes through quickly but bugs and lags and things can and do happen sometimes. No need to worry about the money in any case, either they’ll manage to nudge the order into existence or refund and try to figure out what went wrong. Just don’t try to order again until you got a response. I hope your order wasn’t too urgent but usually CS replies within 24 hours, so you should hear from them soon.